There are essentially two types of feeders for use for after-treatment of sheets of paper in printing machines and copiers, viz. friction feeders and vacuum feeders. In friction feeding devices or feeders, individual sheets are picked from piles of sheets by the fact that a rotary feeding roll is abutted against and pulls the top sheet from the pile, wherein a subjacent separations or friction block normally retains subjacent sheets of the pile. Friction feeders are robust and in general reliable in operation, but occasionally more than one sheet at a time may happen to be picked mistakenly. The feeding rolls may also leave marks in the sheets. In vacuum feeders, sheets are picked from piles by the fact that the top sheet of the pile is sucked against a conveyor belt for transportation of the sheet to subsequent further processing. The vacuum feeder has not the disadvantages mentioned above of the friction feeder, but the function thereof is more sensitive and a vacuum feeder is considerably more expensive than a friction feeder.
A problem in friction feeding devices or friction feeders is to pull the uppermost sheet of paper in a straight direction, since the height of the stack of papers may not be uniform across the stack. This is the case when one side area of the stack of papers is higher than the opposite side area. This can be due to the printing process used giving the sheets of paper a wave shape or for other reasons.